In automobiles and other vehicles, steel plates used in the bodies are reinforced by various means. For example, relatively large and flat but thin exterior plates such as those for roofs, fenders, hoods, trunks, quarter-panels and doors are reinforced by attaching metallic reinforcing parts to the inside of these exterior plates by means of spot welding and adhesive so as to impart proper stiffness to the plates, and thereby the plates withstand external forces.
The above method, however, is disadvantageous in that the great weights of the metallic reinforcing parts cause the exterior plates, which have been designed to have small thicknesses based on the trend toward weight reduction in vehicle bodies, to have increased weights after the reinforcement, and that the use of the metallic reinforcing parts heightens production costs and necessitates a complicated process for attaching the reinforcing parts. In addition, the plates reinforced by metallic reinforcing parts are apt to rust at the spot-welded areas, and a sufficient vibration-damping effect cannot be obtained by the reinforcement with the metallic reinforcing parts.
For the above reasons, reinforcement with resins is being adopted. The following two techniques have been proposed as methods for reinforcement with resins, as described, e.g., in U.S. Pat. No. 4,374,890, JP-A-57-89952, JP-A-1-221236, JP-A-U-59-139320, JP-B-58-50661 and JP-B-60-37837. (The term "JP-A" as used herein means an unexamined published Japanese patent application; the term "JP-B" as used herein means an examined Japanese patent publication; and the term "JP-A-U" as used herein means an unexamined published Japanese utility model application.)
The first method is to coat or adhere polymeric materials, such as asphalt rubbers, butyl rubbers, epoxy resins, phenolic resins, acrylic resins and unsaturated polyester resins, on the back surface of car body exterior plates.
The second method is to reinforce car body exterior plates by means of a laminate consisting of two resin layers.
The above laminate is a resinous reinforcing material composed of a first resin layer containing a reinforcing substrate incorporated therein so as to have a high reinforcing effect and a second resin layer which has a low stiffness and serves to prevent deformation of the steel plates.
The first resin layer in the above reinforcing material may contain a foaming agent which decomposes upon heating to produce foam, thereby giving a resinous reinforcing sheet which increases in thickness through heat cure and, hence, has a heightened reinforcing effect.
The above-described first method has advantages that if a resin material having a high tensile modulus and a high adhesion strength is selected as the polymeric material and applied at a large thickness, a considerable reinforcing effect is obtained, and that, in comparison to the reinforcement with metallic reinforcing materials, the weight of the reinforcing material is small and the reinforcing process is simple.
However, almost all the polymeric materials having high tensile moduli or high adhesion strengths are based on thermosetting resin compositions, and such polymeric materials contract through cure to have a residual stress which cause the thus-reinforced exterior plate to suffer from deformation (depressed areas), resulting in a vehicle body having poor flat-top characteristics that severely impair the commercial value of the final product. To avoid such a vital problem, resin compositions having relatively low moduli can only be used, so that a sufficient reinforcing effect cannot be attained.
The second method can attain, to some extent, both the reinforcement of steel plates and the prevention of plate deformation which are contradictory to each other. However, this method has the following disadvantages.
Although there is an advantage in the above-described second method that the stiffness of the cured resinous reinforcing sheet can be increased by increasing the thickness of the sheet because the stiffness is in proportion to the third power of the thickness, an increased resin amount results in an increase in car body weight and in production cost. Further, the resinous reinforcing sheets used in the second method generally employ glass cloths as their reinforcing substrates, and these glass cloths cause the following problems. Since general glass cloths are soft and the resin used for producing reinforcing sheets should also be soft because it is required to have tackiness, the resulting reinforcing adhesive sheets are so pliable that the application properties thereof in automobile-production lines are impaired. In particular, where a reinforcing adhesive sheet is used in summer (i.e., at relatively high temperature) or should be applied one-handed, the sheet is required to have a certain degree of stiffness. Hence, there is now a desire for a reinforcing adhesive sheet having moderate stiffness and improved application properties. Furthermore, because the reinforcing adhesive sheets used in the second method are so pliable, air is apt to be included between the sheets and the plates during application of the sheets and this may cause blisters in the sheets after heat cure. Moreover, when the reinforcing adhesive sheets are cut into predetermined shapes, the glass cloths fray and glass fibers are released therefrom, causing pollution of coatings.
A further problem of the second method is that although it has become possible to increase the thickness of the heat-cured reinforcing sheet without increasing the thickness of the uncured reinforcing sheet by the incorporation of a foaming agent in the first resin layer, foaming of the resin results in a decrease in the strength of the first resin layer, so that only a slight improvement in reinforcing property can actually be attained by increasing the thickness.
Still a further problem of the second method is that since the reinforcing adhesive sheet has a two-layer structure composed of the first and second resin layer, resins should be used in large quantities and the process for producing the sheet involves many steps, resulting in a low production efficiency and high production costs.